Method for producing barium titanate powder

ABSTRACT

The invention provides a method for producing barium titanate powder comprising the steps of: adding an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m 2 /g to 400 m 2 /g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.

TECHNICAL FIELD

The present invention relates to a method for producing barium titanate powder, more specifically to a method for producing barium titanate powder which is comprised of uniform fine particles, high in tetragonality, and superior in dispersibility.

BACKGROUND ART

In recent years, a substantial improvement of the properties of elements configuring the electronic devices and the starting materials for manufacturing the electronic devices has been strongly demanded with a reduction in size, an enhancement of performance, and a reduction in weight of various electronic devices.

For example, a multilayer ceramic capacitor (MLCC) has been increasingly strongly demanded to reduce the layer thickness. Therefore, barium titanate for use in dielectric layers in the MLCC has been increasingly demanded to be uniform fine particles and to be high in tetragonality and to be superior in dispersibility, for example.

Heretofore, as a method for producing barium titanate, a solid phase method, an oxalic acid method, a sol-gel method, and the like are known. However, in order to produce barium titanate fine particles, particularly those having a particle diameter of about 250 nm or less, preferably about 150 nm or less, for satisfying the recent demand for the reduction in layer thickness of the MLCC, a hydrothermal process which is a wet process is advantageous. Since the solid phase method and the oxalic acid method include a calcination process, uniform particles are difficult to obtain, and moreover the resulting particles are easy to aggregate so that fine particles are difficult to obtain. The sol-gel method employs an expensive alkoxide as the raw material, and therefore the sol-gel method has a problem in the production cost.

Various methods for producing barium titanate by the hydrothermal process are heretofore already known. By way of example, a method is mentioned in which an aqueous solution of barium salt is added to and reacted with a slurry of hydrous titanium oxide in the presence of a carboxylic acid to produce barium titanate core particles, the slurry containing the barium titanate core particles thus obtained is subjected to hydrothermal treatment to obtain spherical barium titanate particles, and then the obtained spherical barium titanate particles are calcined at a temperature of 800° C. to 1200° C. (Patent Literature 1).

According to this method, barium titanate fine particles having a relatively large BET specific surface area and a high tetragonality can be obtained. However, as the method requires calcination of barium titanate core particles, it is difficult to obtain particles having a uniform particle diameter. It is also difficult to obtain particles of fine particles because of aggregation of particles. The use of such barium titanate as a dielectric material in the manufacturing of MLCC poses difficulty in satisfying the demand of reducing the layer thickness of MLCC.

CITATION LIST Patent Literature

Patent Literature 1: JP 2002-211926A

SUMMARY OF INVENTION Technical Problem

The invention has been completed in order to solve the above-mentioned problems involved in the production of barium titanate powder. It is an object of the invention to provide a method for producing barium titanate powder which is comprised of uniform fine particles high in tetragonality and superior in dispersibility.

Solution to Problem

The invention provides a method for producing barium titanate powder comprising the steps of:

adding an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and

subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.

A particularly preferable method for producing barium titanate powder according to the invention comprises the steps of:

adding an aqueous solution of titanium tetrachloride and an aqueous solution of an alkali at the same time to water maintained at a temperature of 45° C. to 65° C. while maintaining the temperature of the resulting reaction mixture in the range of 45° C. to 65° C. and maintaining the pH thereof in the range of 1.5 to 3.5 to simultaneously neutralize the titanium tetrachloride with the alkali thereby to provide an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction;

adding the aqueous slurry of anatase hydrous titanium oxide thus obtained to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and

subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.

According to the invention, it is preferred that in the step of hydrothermal treatment mentioned above, that is, in the step of hydrothermally treating the barium titanate precursor to provide barium titanate particles, the hydrothermal treatment temperature is 250° C. or less.

Advantageous Effects of Invention

According to the method of the invention, there is obtained barium titanate powder which is comprised of uniform fine particles high in tetragonality and superior in dispersibility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron photomicrograph (50000 magnifications) of an example of barium titanate obtained by the method of the invention.

DESCRIPTION OF EMBODIMENTS

The method for producing barium titanate powder of the invention comprises the steps of:

adding an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and

subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.

When the BET specific surface area of the anatase hydrous titanium oxide used is larger than 400 m²/g, the resulting barium titanate is inferior in tetragonality. When the BET specific surface area of the anatase hydrous titanium oxide used is smaller than 200 m²/g, the anatase hydrous titanium oxide has a high crystallinity, but on the other hand, it has a poor reactivity with barium hydroxide with the consequence that the barium titanate particles obtained are inferior in tetragonality.

In particular, according to the invention, the anatase hydrous titanium oxide used has a BET specific surface area preferably in the range of 200 m²/g to 350 m²/g, and more preferably in the range of 220 m²/g to 330 m²/g.

Further, when the anatase hydrous titanium oxide used has a half width of diffraction peak of (101) plane smaller than 2.3° as measured by X-ray diffraction, it has a high crystallinity, but has a poor reactivity with barium hydroxide, and therefore the resulting barium titanate particles are low in tetragonality. On the other hand, when the anatase hydrous titanium oxide used has a half width larger than 5.0°, the resulting barium titanate particles are low in tetragonality.

In particular, the anatase hydrous titanium oxide used in the invention has a half width of diffraction peak of (101) plane preferably in the range of 2.3° to 4.0° as measured by X-ray diffraction, and more preferably in the range of 2.3° to 3.5°.

As described above, the aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction is obtained preferably by adding an aqueous solution of titanium tetrachloride and an aqueous solution of an alkali each maintained at a temperature in the range of 45° C. to 65° C. at the same time to water maintained at a temperature of 45° C. to 65° C. beforehand to simultaneously neutralize the titanium tetrachloride with the alkali while maintaining the pH of the resulting reaction mixture in the range of 1.5 to 3.5, preferably in the range of 2.0 to 3.5.

Therefore, according to the invention, the most preferred method for producing barium titanate powder comprises the steps of:

adding an aqueous solution of titanium tetrachloride and an aqueous solution of an alkali at the same time to water maintained at a temperature of 45° C. to 65° C. while maintaining the temperature of the resulting reaction mixture in the range of 45° C. to 65° C. and maintaining the pH thereof in the range of 1.5 to 3.5 to simultaneously neutralize the titanium tetrachloride with the alkali thereby to provide an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction;

adding the aqueous slurry of anatase hydrous titanium oxide thus obtained to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and

subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.

According to the invention, as described above, the aqueous slurry of anatase hydrous titanium oxide is obtained preferably by simultaneously neutralizing titanium tetrachloride with an alkali. Herein, the simultaneous neutralization of the titanium tetrachloride with the alkali refers to simultaneously adding an aqueous solution of titanium tetrachloride and an aqueous solution of alkali into a container containing water and mixing them in the container to neutralize the titanium tetrachloride with the alkali.

In the simultaneous neutralization of the titanium tetrachloride with the alkali described above, the concentration of the hydrous titanium oxide in the slurry when the simultaneous neutralization has been completed is not particularly limited, and it is usually preferred that it is in the range of 10 g/L to 50 g/L in terms of TiO₂. In case that the concentration of the hydrous titanium oxide in the slurry is higher than 50 g/L in terms of TiO₂ when the simultaneous neutralization has been completed, amorphous hydrous titanium oxide is likely to generate, and therefore it becomes difficult to obtain anatase hydrous titanium oxide. On the other hand, in case that the concentration of the hydrous titanium oxide in the slurry is lower than 10 g/L in terms of TiO₂ when the simultaneous neutralization has been completed, the productivity is likely to be poor.

Further, in the simultaneous neutralization of the titanium tetrachloride with the alkali described above, the temperatures of water, the aqueous solution of titanium tetrachloride, and the aqueous solution of the alkali do not need to be all the same, but the temperatures are preferably close to each other, most preferably all the same, in particular.

As the aqueous solution of the alkali solution, an aqueous solution of an alkali metal hydroxide such as sodium hydroxide and potassium hydroxide, ammonia, and the like are preferably used. In place of the aqueous solution of alkali, a solid alkali compound may be directly added.

Even if the pH of the resulting reaction mixture is in the range of 1.5 to 3.5 in the simultaneous neutralization of the titanium tetrachloride with the alkali described above, when the neutralization temperature is lower than 45° C., the anatase hydrous titanium oxide cannot be obtained, and the BET specific surface area of the resulting hydrous titanium oxide exceeds 400 m²/g.

When the neutralization temperature is higher than 65° C., the titanium tetrachloride is hydrolyzed, and rutile hydrous titanium oxide is likely to generate or the half width of the hydrous titanium oxide becomes smaller than 2.3°. The rutile hydrous titanium oxide has a poor reactivity with barium hydroxide and the barium titanate particles obtained are inferior in tetragonality. Also when the half width of the hydrous titanium oxide is smaller than 2.3°, the barium titanate particles obtained are inferior in tetragonality.

On the other hand, even if the neutralization temperature is in the range of 45° C. to 65° C., when the pH of the resulting reaction mixture is higher than 3.5, the anatase hydrous titanium oxide cannot be obtained and the BET specific surface area of the resulting hydrous titanium oxide exceeds 400 m²/g. When using such hydrous titanium oxide, the barium titanate particles obtained are inferior in tetragonality.

When the pH of the resulting reaction mixture is lower than 1.5, a large amount of chloride ions remain as impurities in the hydrous titanium oxide produced, and as a result, the reactivity thereof with barium hydroxide deteriorates and thus the barium titanate particles obtained are inferior in tetragonality.

Thus, according to the invention, titanium tetrachloride is simultaneously neutralized with the alkali in water to provide hydrous titanium oxide, the resulting slurry is filtered and washed with water to remove therefrom the chloride ions and the like generated in the neutralization, and the resulting cake is dispersed in water, thereby the aqueous slurry of hydrous titanium oxide is obtained which can be preferably used in the step of obtaining an aqueous slurry of the barium titanate precursor.

The aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction is added to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide as described above to provide an aqueous slurry of barium titanate precursor.

The barium titanate precursor obtained by the reaction of the barium hydroxide with the hydrous titanium oxide is usually comprised of fine particles of barium titanate having a BET specific surface area in the range of 50 m²/g to 200 m²/g. Since the reaction between the barium hydroxide and the hydrous titanium oxide is a reaction under normal pressure, the barium titanate precursor obtained has a Ba/Ti ratio smaller than 1, but it has been confirmed by powder X-ray diffraction that the barium titanate precursor obtained has a crystal structure of barium titanate.

In the aqueous solution of barium hydroxide, the solvent may contain a water-soluble organic solvent insofar as the reaction of the barium hydroxide with the hydrous titanium oxide is not adversely affected. As such a water-soluble organic solvent, ethylene glycol, diethylene glycol, polyethylene glycol, and the like can be mentioned, for example.

When obtaining the barium titanate precursor by the reaction of the barium hydroxide with the hydrous titanium oxide, it is preferred that the barium hydroxide and the hydrous titanium oxide are used in such a manner that the Ba/Ti molar ratio is in the range of 1.1 to 3.0 when the addition of the aqueous slurry of anatase hydrous titanium oxide to the aqueous barium hydroxide solution has been completed. In the reaction of the barium hydroxide with the hydrous titanium oxide, when the Ba/Ti molar ratio is smaller than 1.1, the alkalinity of the resulting reaction system is low so that the reactivity between the barium hydroxide and the hydrous titanium oxide deteriorates. When the Ba/Ti molar ratio is higher than 3.0, the reactivity between the barium hydroxide and the hydrous titanium oxide has no problems; however, since the barium hydroxide which does not contribute to the reaction is excessively used, there is caused a problem that the production cost becomes high.

In the step of obtaining the aqueous slurry of the barium titanate precursor described above, the temperature at which the barium hydroxide is reacted with the hydrous titanium oxide is important. Even if the anatase hydrous titanium oxide has a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width in the range of 2.3° to 5.0° as described above, when the temperature of the reaction thereof with the barium hydroxide is lower than 80° C. under normal pressure, barium titanate particles with a high tetragonality cannot be obtained. The upper limit of the reaction temperature is a temperature up to the boiling point of the reaction mixture containing the barium hydroxide.

According to the method of the invention, the barium titanate precursor obtained as described above is subjected to hydrothermal treatment over a period of not less than 24 hours, the resulting slurry is filtered, washed with water, and then dried, to provide the intended barium titanate powder comprised of uniform fine particles superior in tetragonality. If it is intended to synthesize barium titanate only by the hydrothermal treatment without passing through the barium titanate precursor, the reaction of the barium hydroxide with hydrous titanium oxide progresses simultaneously with the particle growth of the resulting barium titanate particles, and, as a result, a large amount of hydroxyl groups are taken into the barium titanate particles, which makes it difficult to obtain barium titanate particles with a high tetragonality.

The aqueous slurry of the barium titanate precursor subjected to hydrothermal treatment preferably contains the barium titanate precursor in a concentration in the range of 0.4 mol/L to 1.0 mol/L in terms of BaTiO₃.

The aqueous slurry of the barium titanate precursor may be subjected to the hydrothermal treatment while maintaining the obtained concentration without condensing or diluting the same.

The aqueous slurry of the barium titanate precursor is put into an autoclave usually as it is, and is then subjected to the hydrothermal treatment at a temperature that exceeds the boiling point thereof under normal pressure and that is usually not more than 250° C., preferably in the range of 105° C. to 250° C.

In the method of the invention, the hydrothermal treatment is performed over a period of 24 hours or more, usually in the range of 24-1000 hours, preferably in the range of 24-500 hours, and most preferably in the range of 24-200 hours. According to the invention, the particle diameter of barium titanate obtained can be controlled by the temperature at which and time for which the hydrothermal treatment is performed.

That is, the particle diameter of the barium titanate increases with increasing temperature at which the hydrothermal treatment is performed, while the particle diameter of the barium titanate increases with increasing time for which the hydrothermal treatment is performed. In particular, according to the present invention, there is obtained barium titanate larger in particle diameter and higher in tetragonality by performing the hydrothermal treatment for a longer time of period. When the hydrothermal treatment is performed for a period of shorter than 24 hours, the resulting particles of barium titanate do not grow sufficiently to be low in tetragonality.

After the aqueous slurry of the barium titanate precursor has been hydrothermally treated in this way, the content in the autoclave is cooled to normal temperature, the obtained slurry is filtered, washed with water, and then dried to obtain barium titanate powder. The drying temperature is usually in the range of 100° C. to 150° C.

The barium titanate powder thus obtained is comprised of uniform fine particles superior in tetragonality, and it usually has a BET specific surface area in the range of 3 m²/g to 70 m²/g, preferably in the range of 4 m²/g to 20 m²/g.

In the invention, the tetragonality of the barium titanate is evaluated on the basis of c/a ratio, which can be determined from powder X-ray diffraction of the barium titanate powder. The barium titanate powder obtained according to the invention has a c/a ratios of 1.008 or more, the tetragonality in that sense. Thus, the barium titanate powder obtained is superior in tetragonality.

As set forth above, because the barium titanate obtained in this way has a high tetragonality only by the hydrothermal treatment of barium titanate precursor, there is no need for the barium titanate obtained to be further calcined and disintegrated.

As described above, the particles of barium titanate obtained by the invention are high in tetragonality and superior in dispersibility. Further, the particles of barium titanate high in tetragonality are also high in dielectricity, and therefore is suitably used as a material for dielectrics. As a consequence, the barium titanate powder obtained by the invention can be suitably used as a dielectric material to meet the demand of reducing the layers thickness in the manufacturing of the MLCC.

The invention is described in detail with reference to Examples, but the present invention is not limited to Examples.

EXAMPLES Example 1 Preparation of Aqueous Slurry of Hydrous Titanium Oxide

500 mL of pure water was put into a beaker and then warmed to 55° C. 350 mL of an aqueous solution of titanium tetrachloride (manufactured by Osaka Titanium Technologies Co., Ltd., 3.8 mol/L in terms of TiO₂) and 7 L of pure water were added at the same time to the water at a rate of 2.5 mL/minute and 50 mL/minute, respectively, while simultaneously an aqueous sodium hydroxide solution having a concentration of 30% by weight was also added to the water, and while maintaining the resulting mixture at a pH of 2.5 and at a temperature of 55° C., to simultaneously neutralize the titanium tetrachloride with the alkali, the sodium hydroxide, thereby an aqueous slurry of hydrous titanium oxide having a concentration of 14 g/L in terms of TiO₂ was obtained.

The aqueous slurry thus obtained was filtered and then washed with water to remove sodium ions and chloride ions therefrom. Pure water was added to the obtained cake to provide an aqueous slurry of anatase hydrous titanium oxide having a concentration of 110 g/L in terms of TiO₂.

(Preparation of Aqueous Slurry of Barium Titanate Precursor)

567 mL of pure water and 959 g of barium hydroxide octahydrate (manufactured by Sakai Chemical Industry Co., Ltd.) were put into a 5 L-capacity reaction vessel, and then heated to 100° C. to dissolve the barium hydroxide octahydrate in water to prepare an aqueous barium hydroxide solution.

The aqueous slurry of the hydrous titanium oxide was added while maintaining the temperature at 100° C. to the aqueous solution of barium hydroxide while maintaining the temperature at 100° C. in 1 hour, and they were reacted at a temperature of 100° C. for 2 hours, thereby an aqueous slurry of barium titanate precursor having a concentration of 0.66 mol/L in terms of BaTiO₃ was obtained. The Ba/Ti molar ratio in the resulting reaction mixture was 2.3 when the addition of the aqueous slurry of hydrous titanium oxide to the aqueous barium hydroxide solution was completed.

(Hydrothermal Treatment of Aqueous Slurry of Barium Titanate Precursor)

The aqueous slurry of the barium titanate precursor having a concentration of 0.66 mol/L in terms of BaTiO₃ obtained above was placed in an autoclave vessel, and then subjected to hydrothermal treatment at 180° C. for 120 hours. Then, the content in the autoclave was allowed to cool to room temperature. The obtained aqueous slurry was filtered, washed with water, and then dried at 130° C. to give barium titanate powder.

FIG. 1 is a scanning electron microphotograph (50000 magnifications) of barium titanate powder obtained above. The invention provides barium titanate powder comprised of uniform fine particles.

Example 2

Barium titanate powder was obtained in the same manner as in Example 1 except that the aqueous slurry of the barium titanate precursor was hydrothermally treated at a temperature of 180° C. for 48 hours.

Example 3

Barium titanate powder was obtained in the same manner as in Example 1 except that the aqueous slurry of the barium titanate precursor was hydrothermally treated at a temperature of 200° C. for 24 hours.

Example 4

Barium titanate powder was obtained in the same manner as in Example 1 except that the aqueous slurry of the barium titanate precursor was hydrothermally treated at a temperature of 160° C. for 168 hours.

Example 5

Barium titanate powder was obtained in the same manner as in Example 1 except that the aqueous slurry of the barium titanate precursor was hydrothermally treated at a temperature of 200° C. for 180 hours.

Example 6

Barium titanate powder was obtained in the same manner as in Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 3.5 and at a temperature of 45° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 170° C. for 96 hours.

Example 7

Barium titanate powder was obtained in the same manner as in Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 2.0 and at a temperature of 45° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 200° C. for 24 hours.

Comparative Example 1 Preparation of Aqueous Slurry of Hydrous Titanium Oxide

500 mL of pure water was put into a beaker, and was maintained at a temperature of 60° C. 350 mL of an aqueous solution of titanium tetrachloride (manufactured by Osaka Titanium Technologies Co., Ltd., 3.8 mol/L in terms of TiO₂) and 7 L of pure water were added at the same time to the water at a rate of 2.5 mL per minute and 50 mL per minute, respectively, while simultaneously an aqueous solution of sodium hydroxide having a concentration of 30% by weight was added to the water and while simultaneously the resulting mixture was adjusted at a pH of 2.0 and at a temperature of 60° C., to simultaneously neutralize the titanium tetrachloride, thereby an aqueous slurry of hydrous titanium oxide having a concentration of 14 g/L in terms of TiO₂ was obtained.

The aqueous slurry was filtered and then washed with water to remove sodium ions and chloride ions therefrom. Pure water was added to the obtained cake to provide an aqueous slurry of anatase hydrous titanium oxide having a concentration of 110 g/L in terms of TiO₂.

(Preparation of Aqueous Slurry of Barium Titanate Precursor)

567 mL of pure water and 959 g of barium hydroxide octahydrate (manufactured by Sakai Chemical Industry Co., Ltd.) were put into a 5 L-capacity reaction vessel, and then heated to 100° C. to dissolve the barium hydroxide octahydrate in water to prepare an aqueous barium hydroxide solution.

The aqueous slurry of the hydrous titanium oxide was added while maintaining the temperature at 100° C. to the aqueous solution of barium hydroxide while maintaining the temperature at 100° C. in 1 hour, and they were reacted at a temperature of 100° C. for 2 hours, thereby an aqueous slurry of barium titanate precursor having a concentration of 0.66 mol/L in terms of BaTiO₃ was obtained. The Ba/Ti molar ratio in the resulting reaction mixture was 2.3 when the addition of the aqueous slurry of hydrous titanium oxide to the aqueous barium hydroxide solution was completed.

(Hydrothermal Treatment of Aqueous Slurry of Barium Titanate Precursor)

The aqueous slurry of the barium titanate precursor having a concentration of 0.66 mol/L in terms of BaTiO₃ obtained above was placed in an autoclave vessel, and then subjected to hydrothermal treatment at 190° C. for 0.5 hours. Then, the content in the autoclave was allowed to cool to room temperature. The obtained aqueous slurry was filtered, washed with water, and then dried at 130° C. to give barium titanate powder.

Comparative Example 2

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a temperature of 50° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 180° C. for 20 hours.

Comparative Example 3

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a temperature of 50° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 205° C. for 2 hours.

Comparative Example 4

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 3.0, reaction of hydrous titanium oxide and barium hydroxide was performed at a temperature of 100° C. for 5 hours, and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 130° C. for 0.5 hours.

Comparative Example 5

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a temperature of 50° C., the reaction of hydrous titanium oxide and barium hydroxide was performed at a temperature of 80° C. for 5 hours, and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 200° C. for 2 hours.

Comparative Example 6

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 3.0, the reaction of hydrous titanium oxide and barium hydroxide was performed at a temperature of 95° C. for 5 hours, and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 180° C. for 0.5 hours.

Comparative Example 7

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a temperature of 50° C., the reaction of hydrous titanium oxide and barium hydroxide was performed at a temperature of 100° C. for 2 hours, and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 110° C. for 2 hours.

Comparative Example 8

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 5.0 and at a temperature of 20° C., the reaction of hydrous titanium oxide and barium hydroxide was performed at a temperature of 70° C. for 2 hours, and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 150° C. for 160 hours.

Comparative Example 9

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 2.5 and at a temperature of 55° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 250° C. for 2 hours.

Comparative Example 10

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a temperature of 45° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 180° C. for 20 hours.

Comparative Example 11

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a temperature of 80° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 200° C. for 24 hours.

Comparative Example 12

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 1.5 and at a temperature of 20° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 160° C. for 40 hours.

Comparative Example 13

Barium titanate powder was obtained in the same manner as in Comparative Example 1 except that the simultaneous neutralization of titanium tetrachloride was performed at a pH of 4.0 and at a temperature of 40° C., and the hydrothermal treatment of barium titanate precursor was performed at a temperature of 170° C. for 24 hours.

The reaction conditions in the above-described Examples 1-7 and Comparative Examples 1-13, i.e., the pH and temperature at which titanium tetrachloride was simultaneously neutralized with sodium hydroxide, the reaction temperature at which and the reaction time for which the hydrous titanium oxide was reacted with barium hydroxide to provide the barium titanate precursor, and the temperature at which and the time for which the aqueous slurry of barium titanate precursor was hydrothermally treated are shown in Table 1.

The properties of the hydrous titanium oxide used in Examples 1-7 and Comparative Examples 1-13, and the properties of the barium titanate powder obtained in Examples 1-7 and Comparative Examples 1-13 are shown in Table 2.

(Crystal Structure of Hydrous Titanium Oxide)

A small amount of the aqueous slurry of hydrous titanium oxide obtained by simultaneously neutralizing titanium tetrachloride with sodium hydroxide was withdrawn, filtered and washed with water, and dried to provide powder. The crystal structure of the powder was examined using a powder X-ray diffraction apparatus (RINT-TTR III, manufactured by Rigaku Corporation, Radiation source: CuKα).

Further, the half width was calculated from the peak of (101) plane and was used as the index of the crystallinity of the hydrous titanium oxide. As shown in Table 2, the peak of the hydrous titanium oxide in each of Comparative Examples 8, 12 and 13 was found to be very broad, and thus the hydrous titanium oxides obtained therein were judged to be amorphous.

(c/a Ratio of Barium Titanate)

The barium titanate powder was subjected to powder X-ray diffraction using a powder X-ray diffraction apparatus (RINT-TTR III, manufactured by Rigaku Corporation, Radiation source: CuKα), and then the c/a ratio was determined using the WPPF method.

(Specific Surface Areas of Hydrous Titanium Oxide Powder and Barium Titanate Powder)

The specific surface area of each of the hydrous titanium oxide powder and the barium titanate powder was measured by a single point BET method using a fully automatic specific surface area meter (HM Model-1220, manufactured by Mountech Co., Ltd.), after degassing at 205° C. for 30 minutes.

TABLE 1 Preparation of Preparation of Hydrothermal Hydrous Barium Titanate Treatment Titanium Oxide Precursor Temperature Temper- Temper- Temper- ature ature Time ature Time pH (° C.) (° C.) (h) (° C.) (h) Example 1 2.5 55 100 2.0 180 120 Example 2 2.5 55 100 2.0 180 48 Example 3 2.5 55 100 2.0 200 24 Example 4 2.5 55 100 2.0 160 168 Example 5 2.5 55 100 2.0 200 180 Example 6 3.5 45 100 2.0 170 96 Example 7 2.0 45 100 2.0 200 24 Comparative 1 2.0 60 100 2.0 190 0.5 Comparative 2 2.0 50 100 2.0 180 20 Comparative 3 2.0 50 100 2.0 205 2.0 Comparative 4 3.0 60 100 5.0 130 0.5 Comparative 5 2.0 50 80 5.0 200 2.0 Comparative 6 3.0 60 95 5.0 180 0.5 Comparative 7 2.0 50 100 2.0 110 2.0 Comparative 8 5.0 20 70 2.0 150 160 Comparative 9 2.5 55 100 2.0 250 2.0 Comparative 10 2.0 45 100 2.0 180 20 Comparative 11 2.0 80 100 2.0 200 24 Comparative 12 1.5 20 100 2.0 160 40 Comparative 13 4.0 40 100 2.0 170 24

TABLE 2 Hydrous Titanium Oxide Barium Titanate Specific Specific Surface Half Surface Area Crystal Width Area (m²/g) Structure*⁾ (Degrees) (m²/g) c/a Example 1 255 A 2.5 7.4 1.010 Example 2 255 A 2.5 10.5 1.008 Example 3 249 A 2.4 11.7 1.008 Example 4 249 A 2.4 10.1 1.009 Example 5 255 A 2.5 4.9 1.010 Example 6 322 A 3.1 8.7 1.008 Example 7 284 A 3.0 8.6 1.008 Comparative 1 270 A 2.5 20.6 1.006 Comparative 2 256 A 2.5 11.8 1.007 Comparative 3 256 A 2.5 17.1 1.006 Comparative 4 226 A 2.4 48.3 1.005 Comparative 5 256 A 2.5 21.7 1.006 Comparative 6 226 A 2.4 25.2 1.006 Comparative 7 256 A 2.5 68.4 1.003 Comparative 8 409 Am 6.3 8.1 1.007 Comparative 9 255 A 2.5 17.0 1.006 Comparative 10 284 A 3.0 13.7 1.007 Comparative 11 158 A 1.6 10.2 1.007 Comparative 12 401 Am 5.5 15.9 1.006 Comparative 13 422 Am 6.5 14.7 1.006 *⁾A represents “anatase”; Am represents “amorphous”.

As shown in Table 2, the barium titanate powder obtained by the method of the invention has a specific surface area in the range of 4 to 20 m²/g and a c/a ratio of 1.008 or more. Thus, the barium titanate powder obtained by the method of the invention is comprised of fine particles superior in tetragonality.

Further, the method of the invention needs no calcination of barium titanate powder obtained by hydrothermal treatment of barium titanate precursor, and accordingly the method of the invention usually provides barium titanate powder superior in dispersibility without disintegration of the resulting barium titanate powder. 

The invention claimed is:
 1. A method for producing barium titanate powder comprising the steps of: adding an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.
 2. The method according to claim 1, wherein in the step of subjecting the barium titanate precursor to hydrothermal treatment to provide barium titanate, the hydrothermal treatment is performed at a temperature of 250° C. or less.
 3. The method according to claim 1 comprising the steps of: adding an aqueous solution of titanium tetrachloride and an aqueous solution of an alkali to water maintained at a temperature of 45° C. to 65° C. at the same time while maintaining the temperature of the resulting reaction mixture in the range of 45° C. to 65° C. and maintaining the pH thereof in the range of 1.5 to 3.5 to simultaneously neutralize the titanium tetrachloride with the alkali thereby to provide an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m²/g to 400 m²/g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction; adding the aqueous slurry of anatase hydrous titanium oxide thus obtained to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide solution at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.
 4. The method according to claim 3, wherein in the step of subjecting the barium titanate precursor to hydrothermal treatment to obtain barium titanate, the hydrothermal treatment is performed at a temperature of 250° C. or less. 